Massless scalar field in Robertson-Walker space-time     

S. D. Maharaj  R. Maartens  A. Beesham 《Astrophysics and Space Science》1990,167(2):329-333

Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x ^l ). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t).  相似文献   

Modeling X-Ray Photoionized Plasmas Produced at the Sandia Z-Facility     

P. A. M. Van Hoof  M. E. Foord  R. F. Heeter  J. E. Bailey  H.-K. Chung  M. E. Cuneo  W. H. Goldstein  V. Jonauskas  F. P. Keenan  R. Kisielius  D. A. Liedahl  C. Ramsbottom  S. J. Rose  P. T. Springer  R. S. Thoe 《Astrophysics and Space Science》2005,298(1-2):147-153

In experiments at the high-power Z-facility at Sandia National Laboratory in Albuquerque, New Mexico, we have been able to produce a low density photoionized laboratory plasma of Fe mixed with NaF. The conditions in the experiment allow a meaningful comparison with X-ray emission from astrophysical sources. The charge state distributions of Fe, Na and F are determined in this plasma using high resolution X-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter ξ = 20–25 erg cm s⁻¹ under nearly steady-state conditions. First comparisons of the measured charge state distributions with X-ray photoionization models show reasonable agreement, although many questions remain.  相似文献   

Characterization of soil's structural crust by spectral reflectance in the SWIR region (1.2–2.5 μm)     

N. Goldshleger  E. Ben-Dor  Y. Benyamini  M. Agassi  & D. G. Blumberg 《地学学报》2001,13(1):12-17

Structural crust is a thin layer formed on the soil surface after a rainstorm. The crust is the result of a physical segregation and rearrangement of soil particles in a way that affects some of the soil properties, such as infiltration, runoff and soil erosion. In practice, there is no rapid, in situ method for monitoring, assessing and mapping crust intensity and quality. In this study, a controlled spectral investigation of the structural crust across the NIR–SWIR spectral region was conducted on three selected Israeli soils, to study the potential of reflectance radiation to detect structural crust in soils. Two major factors served as the driving forces for this study: (1) there is no valid method for in situ assessment of the crust's characteristics in the agriculture field, and (2) the crust might bias thematic remote sensing of soils, because the thin layer of crust blocks photon–matter interaction, which represents the relevant soil body. Through the use of a laboratory rainfall simulator and a sensitive spectrometer, it was revealed that for three selected soils, significant spectral differences occurred between the crust and its bulk soil. The spectral information was found to be related to changes in particle size distribution and texture at the surface of the soil. This conclusion was based on indications of absorption of OH in clay lattice, OH in adsorbed water and CO₃ in carbonates. It was concluded that the structural crust is a phenomenon that should not be ignored by remote-sensing users. In fact, in the field of agriculture, the spectral properties of crust can be used as tools for estimating the crust's intensity.  相似文献   

Neutron background in large-scale xenon detectors for dark matter searches     

M. J. Carson  J. C. Davies  E. Daw  R. J. Hollingworth  V. A. Kudryavtsev  T. B. Lawson  P. K. Lightfoot  J. E. McMillan  B. Morgan  S. M. Paling  M. Robinson  N. J. C. Spooner  D. R. Tovey 《Astroparticle Physics》2004,21(6):667-687

Simulations of the neutron background for future large-scale particle dark matter detectors are presented. Neutrons were generated in rock and detector elements via spontaneous fission and (α,n) reactions, and by cosmic-ray muons. The simulation techniques and results are discussed in the context of the expected sensitivity of a generic liquid xenon dark matter detector. Methods of neutron background suppression are investigated. A sensitivity of 10⁻⁹–10⁻¹⁰ pb to WIMP-nucleon interactions can be achieved by a tonne-scale detector.  相似文献   

On the survivability of an enantiomeric excess of amino acids in comet nuclei during the decay of26Al and other radionuclides     

Rafael Navarro-González  Alfredo Romero 《Astrophysics and Space Science》1996,236(1):49-60

We present a computer model calculation for the racemization of a possible excess of amino acids in the icy fraction of comet nuclei bring about by ionizing radiation released during the decay of²⁶Al,⁴⁰K,²³⁵U,²³⁸U and²³²Th. The model takes into account a total of 110 chemical reactions, of which 91 are needed to explain the radiation chemical processing of the major constituents of comet nuclei (Navarro-Gonzálezet al., 1992) and 19 are necessary to simulate the radiolysis of glycine/alanine mixtures in aqueous solutions (Navarro-Gonzálezet al., 1994 and 1996). It is predicted that an enantiomeric excess of alanine would not be destroyed by radioracemization during the decay of embedded radionuclides. Nevertheless, this enantiomeric excess could be attenuated by the formation of racemic amino acids in the interior of comet nuclei as a result of the radiation-induced polymerization of HCN.  相似文献   

Optical and X-Ray Observations of the Symbiotic System Ag Draconis During Quiescence and Outburst     

Roberto F. Viotti  Rosario González-Riestra  Takashi Iijima  Stefano Bernabei  Riccardo Claudi  Jochen Greiner  Michael Friedjung  Vito Francesco Polcaro  Corinne Rossi 《Astrophysics and Space Science》2005,296(1-4):435-439

We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations. We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and X-ray observations during the 2003 outburst. Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes.  相似文献   

Effects of clumping on temperature – I. Externally heated clouds     

S. D. Doty  R. A. Metzler  M. L. Palotti 《Monthly notices of the Royal Astronomical Society》2005,362(2):737-747

We present a study of radiative transfer in dusty, clumpy star-forming regions. A series of self-consistent, 3D, continuum radiative transfer models are constructed for a grid of models parametrized by central luminosity, filling factor, clump radius and face-averaged optical depth. The temperature distribution within the clouds is studied as a function of this parametrization. Among our results, we find that: (i) the effective optical depth in clumpy regions is less than in equivalent homogeneous regions of the same average optical depth, leading to a deeper penetration of heating radiation in clumpy clouds, and temperatures higher by over 60 per cent; (ii) penetration of radiation is driven by the fraction of open sky (FOS) – which is a measure of the fraction of solid angle along which no clumps exist; (iii) FOS increases as clump radius increases and as filling factor decreases; (iv) for values of   FOS >0.6–0.8  the sky is sufficiently 'open' that the temperature distribution is relatively insensitive to FOS; (v) the physical process by which radiation penetrates is preferentially through streaming of radiation between clumps as opposed to diffusion through clumps; (vi) filling factor always dominates the determination of the temperature distribution for large optical depths, and for small clump radii at smaller optical depths; (vii) at lower face-averaged optical depths, the temperature distribution is most sensitive to filling factors of 1–10 per cent, in accordance with many observations; (viii) direct shadowing by clumps can be important for distances approximately one clump radius behind a clump.  相似文献   

Shock Propagation Through Multiphase Media*     

R. J. R. Williams  D. L. Youngs 《Astrophysics and Space Science》2005,298(1-2):191-196

This paper presents two and three dimensional simulations of the interaction of shocks with media with large numbers of dense inclusions. An approximate model of the interaction of a starburst wind with the surrounding galactic ISM illustrates issues which must be addressed in global models of ISM dynamics. As a step towards developing the sub-grid model of multiphase turbulence, we define and study a form of ‘multiphase Riemann problem’. This allows us to develop macroscopic characteristics of the flows which may be compared to such subgrid models.  相似文献   

Alignment of Fabry-Pérot Interferometer Plates in a Solar Vector Magnetograph     

D.L. Mickey 《Solar physics》2004,220(1):21-27

I describe a method for quickly and accurately determining the plate parallelism in a tunable Fabry-Pérot interferometer. The method takes advantage of the fact that our Fabry-Pérot is installed in a spectropolarimeter: we measure profiles of solar or telluric absorption lines in three differently polarized telescope subapertures and use the residual polarization profiles to determine the plate parallelism error. An example of the error as a function of time during the observing day is also shown; it changes in a consistent way with telescope hour angle but the drift is enough to require frequent adjustment.  相似文献   

Current plate motions   总被引：57，自引：0，他引：57  

C. DeMets  R. G. Gordon  D. F. Argus  S. Stein 《Geophysical Journal International》1990,101(2):425-478

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